Primary Light Investigation

I was recently asked for my thoughts on developing an investigation for primary students to do with either rainbows or colour perception. I thought I would share with you my ongoing investigation into this investigation.

My first thought was rainbow is a “banned” word in my classroom, I must state that this is because I teach A level physics. At A level we teach how a spectrum is formed mainly from diffraction gratings and prisms. I guess most people and certainly primary students would first come across the phenomenon of dispersion (the spreading of light into different frequency and hence colour) as a rainbow. This is a good starting point to explore some properties of light and the materials it passes through. Hopefully we can introduce the scientifically correct words spectrum and dispersion and still have a pot of golden physics at the end of our rainbow journey. I have a wealth of new and old physics books covering most levels of learning.

So, I set about looking up the forbidden word in the indexes of my books.

As always I first turn to Isaac Asimov and found a great starting phrase to his chapter on the Universe, in his Guide to Science 1, The Physical Sciences

“Humankind’s greatest weapons in the conquest of knowledge are the understanding mind and the inexorable curiosity that drives it on.”

The chapter expands on the invention of the telescope as a use of lenses in 1609.

Being a primary age investigation I thought that Jelly Lenses may be a good thing to investigate.

Interestingly the refracting telescope (Telescope using lenses) has a problem that plagued Newton. Newton in 1666 discovered that light could be split into what he called a spectrum of colours.

You can make a large spectrum using a prism and something like an OHP or projector found in a classroom. The one below was made at last years IOP Cambridge Summer school.

Here the camera CCD actually picks up on some of the infra-red which is also dispersed.

I often think that we teach students that the spectrum is on the screen, when we should explain how it is in all the intervening space. I use a piece of selenite to show this.

A smoke machine could be used to good effect here. Teachers once were able to reach for the nearest chalkboard duster and produce fine particles for scattering light out of the beam.

This effect caused a problem in lenses called chromatic aberration. A spreading of coloured rings around the image formed. He invented the reflecting telescope to avoid this. Joseph von Fraunhofer managed to make a solar spectrum with a prism and noticed missing wavelengths. This dispersion of light in the end became the astronomer’s greatest tool. It can gather information about a star’s temperature, mass, it chemical composition, any winds it produces and if it is orbiting another star.

It is these missing wavelengths that I thought might be what you mention as a way a rainbow can be used to look at air pollution. This is because the missing wavelengths correspond to specific amounts of energy necessary to make things happen to the particles or elements. Examples being, rotation and vibration of the particles that absorb these missing wavelengths. Usually for these processes the missing parts are from the infra-red part of the spectrum and hence would not be seen. However, I think that if information is being gained from rainbows it must be to do with how the particle size of the pollutants in the atmosphere affect the strength or size of the different coloured bands.

This would be hard to reproduce. You can however make rainbows in sunlight with a fine mist sprayer. In the video in the link I used a 69p sprayer from ASDA.

Another way of making rainbows is with a large glass sphere. I use a round bottom flask. This is how the rainbow was first studied many centuries ago.

This produces a visible spectrum and can explain the role of the individual raindrops.

Been looking at some primary investigations as I’ll be heading to the Etihad Stadium (not for football for once!) to attend the Great Science Share on 5th July https://www.greatscienceshare.org/

The day offers an opportunity for primary school students to showcase their science investigations so if you know of any primaries who would be interested please feel free to share the link (and most importantly, offer your ideas and advice as to what to investigate, like the one above!)

I used a round flask to make a single rain drop demo, the light source is a projector. You can get a second rainbow and an alexander’s band. However everything is reversed in this method compared to the rainbow you perceive when looking at the dispersion of sunlight through water droplets. Nobody has ever noticed this so it doesn’t detract from the demo.